The present embodiments relate to calibration for functional imaging. Calibration is provided for quantitative functional imaging.
Positron emission tomography (PET) and single photon emission computed tomography (SPECT) are two types of functional or nuclear imaging. Functional imaging uses a radioisotope or radiotracer to determine metabolic function within a patient. For example, the uptake of the radiotracer by tissues in the body is measured. The emissions from the radiotracer are detected in the functional imaging. The activity concentration (i.e., the concentration of the radiotracer from different locations) is reconstructed from the detected emissions. For quantitative functional imaging, both accurate activity concentration and uptake values are desired. The goal is to provide a global baseline that is free of system (detector and dose calibrator) variability so that any measured change for a patient over time in either quantity is due to metabolic reasons.
The error in the dose applied to the patient introduces a source of error in quantitative functional imaging. A dose value is provided using a measurement by a dose calibrator. The dose value for the liquid isotope applied to the patient may be inaccurate. One source of inaccuracy is contribution from characteristic X-rays. FIG. 1 shows a table of emission spectrum for In111. The table includes energy, the intensity (with % chance of occurring in a given instance of decay and the uncertainty), and the dose for gamma and X-ray emissions. The dose calibrator sensitivity is a highly non-linear function of incident photon or gamma energy. Primary gamma emissions from SPECT tracers are at the minimum of the chamber sensitivity while chamber sensitivity for characteristic X-ray energies of the SPECT tracers are very high. As a result, the dose calibrator measurement of activity includes a larger or comparable amount of energy from characteristic X-rays. For SPECT tracers with high energy gamma emissions in addition to the primary emissions, multiple Compton scattering of the higher energy gamma rays results in dose uncertainty.
To limit energy contribution from characteristic X-rays in dose calibration, a passive shield (e.g., copper jacket) is introduced to differentially attenuate the X-rays relative to the primary emissions. The jacket reduces but does not eliminate the X-rays, attenuates the primary emissions, and has unknown production tolerances, resulting in uncertainties of varying magnitude. For isotopes with significant emissions of characteristic X-rays, the differential attenuation of the X-rays and gamma-rays in the tracer container also creates uncertainty. For isotopes with high energy gamma emissions in addition to primary emissions, the higher efficiency for high energy gamma-rays dues to multiple Compton scattering results in calibration uncertainty.